In conventional art, apparatuses which record long-play content by a recording medium by using data compressed by high-efficiency encoding for digitized image signals, and services which distribute content through digital broadcasting signals from a satellite and a terrestrial station or a network have been put to practical use. In such services, high-efficiency encoding which realizes a large compression rate is required to broadcast or transmit a large quantity of image and sound data which include an enormous data quantity.
To achieve high-efficiency encoding of moving images, encoding methods such as Moving Picture Experts Group phase 2 (MPEG 2), which is an international standard, are used. These encoding methods are methods which compress a data quantity by using correlation between adjacent pixels (spatial direction) of an image signal, and correlation between adjacent frames or fields (time direction).
An image signal of an input frame is divided into processing units, which is called macroblocks, by a brightness signal, and each macroblock includes 16 pixels in a horizontal direction and 16 pixels in a vertical direction. Each divided macroblock data is further divided into two-dimensional blocks, each of which includes 8 pixels in a horizontal direction and 8 pixels in a vertical direction. Each of the divided two-dimensional blocks is subjected to a two-dimensional discrete cosine transformation (DCT), which is a type of discrete orthogonal transformation. Image encoding apparatuses perform quantization for a DCT coefficient which is obtained by the DCT, apply entropy encoding for a quantization result, and thereby generates encoded data.
When an image is compressed by encoding algorithm by DCT and quantization as described above, original data is lost by the quantization, and encoding distortion such as block distortion and mosquito distortion is caused in a decoded image. Therefore, it is impossible in prior art to perform image restoration with high accuracy.